The instrumentation amplifier is a high-accuracy amplifier used for amplifying a weak differential signal, and is used widely in fields of high-accuracy measurement, such as an electronic scale, a pressure measurement, blood analysis, a sphygmomanometer, gas analysis and measurement, and electric energy analysis and measurement.
As shown in FIG. 1, an instrumentation amplifier in the conventional technology is illustrated, for which a closed-loop gain amplification function is achieved by means of two single-output operational amplifiers Av1 and Av2 and three resistors R0 to R2. The instrumentation amplifier has characters of a programmable amplification gain, a low temperature drift, high input impedance and high common-mode rejection ability, on the other hand, the instrumentation amplifier has the following disadvantages.
First, two operational amplifiers are used in the circuit design, resulting in that the cost and power consumption of the instrumentation amplifier are relatively high. Second, the amplification gain of the instrumentation amplifier is determined by the three resistors R0 to R2, the resistors generates a thermal noise, and the greater resistance the resistors have, the greater thermal noise is generated. In this case, a resistor with a lower resistance may be used to design the instrumentation amplifier with the lower noise. However, there will be a higher requirement of a drive capability for the two operational amplifiers Av1 and Av2 if the resistance of the resistor is lower, and as a result, the power consumption of the instrumentation amplifier becomes higher.